Digital or analog equalizers have been widely used in the receiver end of baseband communication systems to combat the intersymbol interference (ISI) caused by frequency dependent loss of metal media or high recording densities in hard disk drives. In recent years, with the increases in data rate among digital integrated circuits, equalizers are also being used at the transmitter or receiver end to compensate for high frequency loss in chip-to-chip communications. Digital or analog finite impulse response (FIR) filters have been used in these equalizers to remove ISI, interference, echoes, and/or noise. Digital FIR filters have been found to achieve high levels of computation precision, flexibility and effective adaptation, ultimately resulting in enhanced performance. Unfortunately, comparatively large circuit area, high power consumption and speed limitations of analog-to-digital converters (ADC) have prevented digital FIR filters from being used in high frequency (e.g., Gbps range) data equalizer designs Today, most analog FIR filters operate at the symbol rate, which means the tap delay interval is the same as the symbol period. They use a lower frequency-sampling clock, but are sensitive to the sampling phase. So timing information is recovered before or simultaneous with start of a data adaptation algorithm. Fractionally spaced FIR filters, on the other hand, are insensitive to the sampling phase and can compensate amplitude distortion with less noise enhancement with a higher sampling frequency.
Electrical equalizers commonly use various techniques to adapt to different transmission lengths, environment thermometer, and process technology. In certain techniques, an error signal relating to the quality of the received data or bit error rate (BER) is generated and used to tune the transfer function of the equalization filter.
There is always the need to develop improved filtering, equalization, and power estimation approaches to support the demand for ever higher data speed transmissions within systems and between systems.